1. Field of the Invention
The present invention relates to a focal plane array imaging device. More particularly, the present invention relates to an imaging device of the focal plane array type which may be employed for visible light, for infrared radiation, or for electromagnetic radiation of other frequencies. The device includes an architecture which allows individual image elements, or pixels, of the device to be randomly accessed individually or in groups. Consequently, the pixels of the device may be scanned individually row by row like a conventional focal plane array imaging device, or the pixels may be accessed individually or in groups to define one or more windows on the array in which an image or images of interest are located. Additionally, the present device allows time-integration at the pixels themselves of a signal originating with electromagnetic radiation from a source which is to be imaged. Thus, low-level image sources which might otherwise be difficult to distinguish from background noise or which would require specialized signal processing may be imaged conveniently with the present imaging device. Also, the time integration feature allows the sensitivity of the array to be dynamically varied and to be different in different windows on the array to accommodate the photovoltaic charge-producing sensitivity of a photoreceptor of the pixels with the particular level of radiation received from particular sources imaged within the different windows on the array.
2. Discussion of Related Technology
Conventional focal plane array imaging devices both for visible light, and for other portions of the electromagnetic spectrum, have been known for some time. These devices are generally of the charge-coupled type or of the direct-injection type. For purposes of convenience and simplicity in description hereinafter, the term "light" or "light-responsive", and other such terms, should be understood to refer to the electromagnetic spectrum in general, and may include both infrared, and ultra-violet radiations, and other frequencies in addition to visible light.
These known conventional focal plane array imaging devices currently are fabricated as arrays of light-responsive elements, or pixels, as thin-film devices generally in a rectangular array of photo-responsive receptors on the face of a semiconductor substrate. The devices are fabricated using conventional thin- film, and other currently-known semiconductor fabrication techniques.
Importantly, all known conventional imaging devices of the focal plane array type are based on an architecture which requires the pixels of the device to be accessed in serial order. That is, the image signal from the pixels is fed out of the imaging device as an analogue or digital data stream representing light levels incident on the pixels individually in a row by row scan of the array. Generally this scan starts at one corner of the rectangular array and proceeds across the row of pixels individually, preceding subsequently across the next or adjacent row of pixels. Of course, scanning every other row of the array with the scan rate being such that two such partial scans of alternate rows are completed in the same time as would be required for a complete scan of adjacent rows is also known to reduce the flicker of a video image (interlacing). With either type of scanning, this type of serial image output signal indicative of a pixel scan is long-familiar from the television technology.
Unfortunately, when it is desired to concentrate attention on a stationary or moving image which resides in a particular part of the image array and occupies a comparatively small portion of the array, a large part of the serial information in the signal stream is of little or no interest. That is, after the last portion of the serial signal stream which includes information about an image of interest is received, almost the entire remaining portion of the array scan (or scan of interlacing alternate rows) must be completed before the scan will return to the area of the array which is of particular interest. Thus, time is lost in acquiring image information from the part of the array which is of most interest. This time loss is the case even is signal acquisition circuitry is employed to acquire and concentrate attention on (i.e., create a window of image out of) the array signal stream.
When it is desired to acquire an image of an image source which is fast-moving or of a low-level source, or both, then the time lost in scanning the entire array, including those areas of the array where image information of little or no interests is located, is a great detriment. This time loss can result, for example, in loss of the image source from the field of view of the imaging system, in confusion of background noise sources for the image source of interest, or both.
One conventional expedient is to simply increase the scanning rate at which the pixels of an imaging device are accessed. This increase of scanning rate results in the scan returning to the area of the array which is of interest more quickly and with less loss of time between scans of the interesting area. However, when the image to be generated is digitized, the analogue-to-digital converters (digitizers) themselves have a finite settling time which limits the rate at which the array can be scanned. The conventional solution to this lack of speed in array scanning is to use plural digitizers in parallel. In this architecture, each of the plural digitizers in sequence is supplied with a portion of the analogue data stream, and is then interrogated for its portion of the resulting digital signal after the digitizer has had time to settle. With the digitizers sharing the load, doubling the scanning speed requires double the number of digitizers. Of course, it is easily seen that this conventional expedient itself has limitations with respect to the cost and complexity of the overall imaging system. As the rate of pixel scanning increases, the number of digitizers required becomes prohibitive.
On the other hand, when it is desirable to image a low-level image source, another limitation of conventional image devices with serial scanning becomes apparent. That is, the sensitivity of the image pixels to the incident radiation from the source cannot be conveniently increased beyond the inherent photovoltaic charge-producing sensitivity of the photodetectors themselves. This sensitivity of the photodetectors is limited because each pixel must not saturate under ordinary operating conditions in the time between scans. With this limitation on the sensitivity of the photodetectors of conventional imaging devices, it is apparent that when a low-level source is to be imaged, the array is low in sensitivity. Thus, the image of a low-level image source may be very difficult to distinguish from the background noise which is also incident on the array.
This problem of a comparatively inflexible image intensity range or sensitivity also manifests itself in image blooming when the conventional arrays are employed to image a high-level source. The image pixels saturate and flood signal onto adjacent pixels so that the output image grows and the true image is lost. That is, the imaging device is unable to differentiate the image of the source from the blooming of signal cascading across the array from saturated pixels. In this case also, the image of interest may be lost from the imaging system because of shortcomings in the imaging array itself.